1. Field of the Invention
The present invention relates to knife valves and, more particularly, to a knife valve adapted for use in paper mills.
2. Description of the Prior Art
Knife valves are used, for instance, in paper mills as they are axially short, and thus light and economical since they are made of stainless steel. Furthermore, the valve's knife blade can cut through solids such as pulp contained in the liquid which circulates therethrough.
On the other hand, these knife valves are bonnet-less in order to prevent, for example, the pulp from jamming the valve stem within the bonnet. Therefore, such knife valves require that the knife blade of the valve which moves up and down and thus in and out of the valve body seals the seat of the valve as well as the knife itself. An elliptical packing chamber disposed around the knife blade is used to seal the knife. The surface which has to be sealed is, in such a case, so large that the seal has to be tightened more and more up to a point where the seat scratches the surface of the knife blade after the valve has been opened and closed a large number of times, which results in the valve leaking. Environmentally and down-time wise, such failures are often unacceptable.
Also, in high performance valves, when the frequency of operation is large and especially if there is a requirement for high pressure operations, that is around 150 PSI, and sealing in two directions, the packing chamber rings of the valve become overcharged with the changing load which results in the valve finally leaking through the packing chamber. It is noted that, when a bidirectional seal is required, gate valves cannot be used because they become jammed with pulp.
In the above knife valves, the knife blade must have a very precise finish in order that it does not cut and damage the elliptical packing seal while ensuring the tightness of the seal.